As an aircraft is approaching an intended landing site, it generally goes into an approach phase in which it seeks to descend along a defined descent path to its landing site. During the approach phase, the altitude of the aircraft is controlled either by its navigation system or by a ground based control system which sends signals to the aircraft to control its altitude. An instrument approach to an airport which relies upon the navigation system for vertical guidance is often referred to as a "non precision" approach, because the navigation system performs calculations to control the altitude of the aircraft, but those calculations are based on sensed barometric pressure which enables the navigation system to approximate, but may not be precise to, the intended descent path of the aircraft. During such a non precision approach, the aircraft descends under the control of the navigation system until the aircraft is at a predetermined distance from its landing site. At that predetermined altitude, the approach depends on the flight crew having sufficient visual contact with the landing site to enable the flight crew to visually land the plane at the landing site.
In contrast, a precision instrument approach to an airport relies upon an instrument landing system (ILS) which continuously transmits information from the landing site to the navigation system during the approach phase and during the landing, to control the altitude of the aircraft relative to the landing site at all times during the approach phase and during the landing, irrespective of whether the flight crew has visual contact with the landing site. An ILS landing does not require the navigation system to calculate the altitude of the aircraft, but under non-ISA temperature conditions it may be useful for the navigation system, or the flight crew, to make a calculation of the actual altitude of the aircraft, to confirm that the aircraft is at the designated altitudes at predetermined points on its descent path to the landing site.
The flight plan entered into the aircraft navigation system indicates the ideal flight path to be flown, both laterally and vertically. Minimum-altitude information, provided by national aviation authorities, has been surveyed and checked to provide ample clearance from terrain and structures. The specific altitude clearance employed varies according to environment and usage. Minimum altitudes during in route operations and over mountains provide wide margins. Minimum altitudes near runways, however, where the aircraft is intentionally maneuvered close to the ground, have very small margins in order to allow the aircraft to maintain a normal descent to the runway.
When these minimum altitudes are entered into the flight plan, then the navigation system can guide the aircraft along the ideal flight path safely above the obstacles. Thus, if the navigation system, and the aircraft's automated guidance system (i.e. the autopilot system) rely upon an altitude input based upon sensed barometric pressure, there is normally enough clearance with the terrain to make up for any inaccuracies due to the actual operating environment of the aircraft.
However, during the approach phase, the if the aircraft is operating under non-ISA temperature conditions, (e.g. if the temperature at the landing site is at or below 0.degree. C.), an altitude calculation based upon the sensed barometric temperature may not be an accurate indication of the actual altitude of the aircraft. Specifically, where landing temperature is significantly below ISA (e.g. 0.degree. C. or less) altitude data based only on barometric pressure would indicate an altitude which is higher than the aircraft's actual altitude, placing the aircraft low in relation to its expected flight path. Hence, as the aircraft descends, the vertical clearances expected by the flight plan may be reduced, possibly without the flight crew's awareness. During a non-precision approach this problem is particularly important, because the navigation system needs to bring the aircraft along a descent path to a point at which the flight crew takes over visually to land the aircraft. If the altitude calculation has a significant variance from the intended altitude, the vertical clearances between the aircraft and the terrain may be eroded to a point where vertical clearances from the terrain present a significant problem. A flight crew can make manual corrections during the approach phase, using tables provided by International Civil Aviation Organization (ICAO) PANS-OPS (Procedures for Air Navigation Services--Aircraft Operations), but such manual corrections require the flight crew to perform mathematical computations quickly during a phase of flight where workload is already quite high. Moreover, the magnitude of the effects of non-ISA temperature on barometric altitude data is not fully understood within the aviation community (e.g. a pilot used to flying in warm weather conditions who is unexpectedly asked to fly a cold weather route would not necessarily understand the magnitude of the effects of non-ISA temperatures on barometric altitude data).